Further penetration of fluctuating renewable energy production requires economic solutions for bulk electricity storage. Today's leading technology is Pumped Hydro Storage (PHS). A possible alternative is Compressed Air Energy Storage (CAES). Whilst PHS requires the right topography i.e. mountains, CAES relies on the presence of specific geological underground structures, such as salt caverns. Other forms of energy storage include batteries and flywheels.
Pumped Heat Electricity Storage (PHES) is an alternative storage technique to both PHS and CAES. During charging a PHES system pumps heat from a low temperature reservoir to a high temperature reservoir, it therefore operates as a heat pump. During discharging the high temperature heat is used to drive a power cycle whilst the residual heat is rejected into the low temperature reservoir. The obvious advantage of such a system is that the electricity is stored only under the form of heat or thermal energy, i.e. it requires only some kind of thermally isolated containment that is independent of geology or topography.
An example of a PHES system is described in EP 2602443. This system may be described as a reversible heat pump. During charging of electricity, a compressor is operated within a heat pump cycle. Heat is absorbed from the ambient and passed to high temperature thermal energy storage (TES). High efficiency is achieved by choosing the upper pressure level of the thermodynamic cycle to be super critical. This allows the transfer of high temperature heat, at nearly constant working fluid heat capacity, to a storage medium such as molten salt that also has a near constant heat capacity.
The disadvantage of supercritical systems is the required high cycle pressures and temperatures in conjunction with the typical need for an organic fluid such as propane or butane that is flammable. These security issues make it difficult to deploy such a system in a domestic situation. Furthermore the cycle is complex to operate, mainly due to the presence of two recuperators and two TES.
German patent 403683 describes an alternative process based on a subcritical cycle which utilities during the discharging cycle heat from water that is available in the environment. The environmental water may be additionally be used to cool condensed working fluid during the charging cycle before throttling and evaporation. The purpose of this is the same namely to reduce irreversibility and thus improve efficiency. However, since the temperature of typical water from the environment will be much smaller than the peak temperature in the hot tank this solution only provides a partial improvement.
Some of drawbacks are at least partially mitigated by the thermoelectric energy storage system described in WO 2010/020480 A2. This solution uses a heat exchanger to transfer thermal energy between a condensable working fluid and a sensible heat thermal storage medium circulating between cold and hot storage tanks. Thermal energy is transferred from the working fluid to the thermal storage medium during a charging cycle and is transferred from the thermal storage medium to the working fluid during a discharging cycle in which electrical energy is generated by expansion of the heated working fluid in a turbine. The condensable working fluid is heated and compressed to a supercritical state during both the charging and discharging cycles and this maximises the round-trip electrical efficiency of the system.
Round-trip electrical efficiency is increased further in the thermoelectric energy storage system described in WO 2011/045282 A2 due to the provision of an internal heat exchanger. The internal heat exchanger preheats the working fluid during both the charging and discharging cycles, thereby maximising system efficiency.
There, however, remains a need for an improved thermal energy storage system which achieves a high round-trip electrical efficiency with minimal capital expenditure.